Distributed Hydrogen Production with Carbon Capture: A Novel Process for the Production of Hydrogen from Biomass

The use of hydrogen as a clean energy carrier is seen by policy makers and industry as a potential way of mitigating climate change arising from the emissions to atmosphere of CO2 arising from the use of fossil fuels. Hydrogen is also, currently, the most desirable fuel for use in the present generation of practicable solid oxide fuel cells. However, to replace fossil fuel with hydrogen requires the hydrogen to be produced either (i) from renewable resources, such as biomass, or (ii) from fossil fuels, with capture and long-term sequestration of the resulting by-product CO2 in the earth. This proposal is concerned with a novel method for the production of hydrogen from biomass, in a clean form suitable for direct use in a fuel cell without substantial gas clean-up. It is also a technique which could lend itself to operation at a range of scales / from small, distributed units, suitable for local sources of biomass or waste (such as in developing countries) to larger, more centralised power stations. Briefly, the process involves:1) The gasification of biomass in CO2 or CO2/steam to syngas containing CO and H2.2) Conversion of the syngas to a pure stream of CO2 and steam by passing it through a packed bed of Fe2O3, where the following reactions variously occur:0.788 CO + 0.947 Fe3O4 = 0.788 CO2 + 3 Fe0.947O (1)0.788 H2 + 0.947 Fe3O4 = 0.788 H2O + 3 Fe0.947O (2)H2 + 3Fe2O3 = 2Fe3O4 + H2O, (3)CO + 3Fe2O3 = 2Fe3O4 + CO2. (4)At an operating temperature of 1173 K, thermodynamic calculations show that eqs. (3) and (4) lie essentially well over to the right where reducing gases are present. Thus, in the packed bed, provided it is sufficiently long, there will be a region of Fe2O3 at the outlet, preceded by a region of Fe3O4. At the entrance to the bed, on the other hand, [CO] and [H2] are high. This means that the Fe3O4 first formed there by reactions (3) and (4) can react further by reactions (1) and (2). Thus, for the Fe3O4 to be reduced to Fe0.947O would require pCO/ pCO2 > 0.49 and pH2/ pH2O > 0.39, which is likely for a typical syngas. Accordingly, at the entrance will be a region of Fe0.947O. As time proceeds, there will, in effect be two fronts moving through the bed: one defining the boundary between Fe0.947O and Fe3O4 and one, nearer the exit, the boundary between Fe3O4 and Fe2O3. The flow of syngas to the packed bed would be stopped just before the Fe3O4 / Fe2O3 front breaks through the bed, to avoid the slip of CO into the outlet stream of gas. Accordingly, the outlet from this bed would be a stream of pure CO2 and some water, which could be condensed out, allowing sequestration of the CO2 if required. A proportion of the CO2 would be recycled to the gasifier. 3) Production of hydrogen. Hydrogen would be generated from the spent bed in 2) by passing steam through it, thus reversing reaction (2). For this to be the case, pH2/ pH2O < 0.39 and so the hydrogen would occur with a front of Fe3O4 propagating from the entrance until it reaches the Fe3O4 left at the end of stage 2).4) Regeneration of Fe2O3. Once the bed is sufficiently converted in 3), air is supplied to the bed to oxidise it back to Fe2O3; the products being depleted air and energy. The heated, depleted air leaves the oxidation reactor at high temperature (ca. 1273 K) and can be used to raise steam.5) Finally, the cycle is repeated, with the supply of syngas re-commenced to the bed regenerated in 4). Hence, by having a number of such beds, arranged in a suitable cyclic operation, it would be possible to operate the gasifier continuously.The overall reaction in the above, assuming that gasification of pure carbon were being undertaken, would be:C(s) + H2O(g) + 2.38 ( 0.21O2(g) + 0.79N2) = CO2(g) + H2(g) + 1.88 N2 Each of the products in this overall reaction would be in a separate stream.